1. Field of the Invention
The present invention relates to filters for use in a frequency band such as a microwave band, antenna duplexers, and communication apparatuses incorporating the same.
2. Description of the Related Art
FIG. 13 shows a conventional trap filter formed by using dielectric resonators (cf. Japanese Unexamined Patent Application Publication No. 63-30003). In a trap filter 201 disclosed in the publication, between an input terminal P1 and an output terminal P2, there are arranged a serial resonance section 203 composed of a capacitor C61 and a dielectric resonator 202, a reactance element 204 electrically connected in parallel to the serial resonance section 203. In FIG. 13, a capacitive reactance element is provided by a capacitor C62 connected to the serial resonance section 203.
In the conventional trap filter 201, however, with the reactance element 204 connected in parallel to the serial resonance section 203, the position of an anti-resonance frequency fa must be on either the frequency-side higher than an attenuation-pole frequency, or the frequency-side lower than the attenuation-pole frequency, but not both. That is, when a capacitor (capacitive reactance element) is used as the reactance element 204, the position of the anti-resonance frequency fa is always on the frequency side higher than the attenuation-pole frequency (see FIG. 14A). In contrast, when an inductor (inductive reactance element) is used as the reactance element 204, the position of the anti-resonance frequency fa is always on the frequency-side lower than the attenuation-pole frequency (see FIG. 14B). Thus, in the conventional trap filter 201, the position of the anti-resonance frequency fa cannot be freely switched between the frequency side higher than the attenuation-pole frequency and the frequency-side lower than that the attenuation-pole frequency.
In addition, as shown in FIG. 15, there is known a conventional band elimination filter 211 with variable attenuation-pole frequency. In FIG. 15, the reference numerals 212 and 213 denote distributed-constant lines having characteristic impedances. The reference numerals C71 and C72 denote capacitors for determining the magnitude of elimination-band attenuation. The reference numerals C73 and C74 denote frequency shifting capacitors for varying two attenuation-pole frequencies. These capacitors C73 and C74 serve to change inductive reactance generated by the distributed-constant lines 212 and 213, respectively. The reference numerals D71 and D72 denote PIN diodes, the reference numerals L71 and L72 denote choke coils. The reference numerals C75 and C76 denote bypass capacitors, the reference numerals r31 and r32 denote voltage-control supplying resistors, the reference numeral L74 denotes a coupling coil, and the reference numerals L73 and L75 denote coils.
When a positive voltage is applied to a voltage control terminal Vc1, the PIN diodes D71 and D72 are switched on. As a result, since the frequency shifting capacitors C73 and C74 are grounded via the PIN diodes D71 and D72, the two attenuation-pole frequencies both become lower (a solid line 215 shown in FIG. 16). When a negative voltage or 0 v is applied to the voltage control terminal Vc1, the PIN diodes D71 and D72 are switched off. As a result, since the capacitors C73 and C74 are open-circuited, the two attenuation-pole frequencies both become higher (a broken line 216 shown in FIG. 16).
In the band elimination filter 211, although the attenuation-pole frequencies can be varied by performing voltage control, it is impossible to significantly change the relationship between the pass band and the attenuation band. That is, in the conventional band elimination filter 211, the position of the attenuation band cannot be freely switched from the frequency-side higher than the pass band to the frequency-side lower than the pass band.
The present invention can provide a filter in which the relationship between a pass band and an attenuation band can be controlled. In addition, the invention can provide an antenna duplexer and a communication apparatus incorporating the filter.
In order to accomplish the above, according to a first aspect of the present invention, there is provided a filter including a trap circuit having a serial resonance section, a capacitive reactance element and an inductive reactance element for making an admittance of the trap circuit substantially zero, both reactance elements being electrically connected in parallel to the serial resonance section, and a switching element connected to one of the capacitive reactance element and the inductive reactance element and being switchable on/off. As the switching element, for example, a PIN diode or a field effect transistor may be used. As the voltage-controllable reactance element, for example, a variable capacitance diode may be used. In addition, the serial resonance section may include a dielectric resonator or a distributed-constant line.
When the switching element is switched on/off, a parallel reactance-element circuit composed of the capacitive reactance element and the inductive reactance element becomes either capacitive or inductive. That is, by the on/off control of the switching element, the position of an anti-resonance frequency fa can be freely switched between the frequency-side higher than an attenuation-pole frequency and the frequency-side lower than the attenuation-pole frequency.
In addition, the voltage-controllable reactance element may be electrically connected to each serial resonance section via a frequency shifting capacitor. With this arrangement, the reactance element is switched on/off by applying a control voltage, by which the frequency shifting capacitor is either grounded or open-circuited. As a result, the attenuation-pole frequency of the trap circuit can be varied.
Furthermore, a plurality of trap circuits having serial resonance sections may be electrically connected to each other by one of an inductive element and a capacitive element. With this arrangement, for example, a band elimination filter is formed. In the band elimination filter, by on/off switching of the switching element, the position of the attenuation band can be freely switched between the frequency-side higher than the pass band and the frequency-side lower than the pass band.
In addition, according to a second aspect of the invention, there is provided a filter including the filter of the first aspect and a band pass filter electrically connected thereto. With this arrangement, by switching the switching element on/off, the position of the anti-resonance frequency of the trap circuit can be freely switched between the frequency-side higher than the pass-band of the band pass filter and the frequency-side lower than that the pass-band of the band pass filter.
According to a third aspect of the invention, there is provided an antenna duplexer including one of the above filters. According to a fourth aspect of the invention, there is provided a communication apparatus incorporating at least one of the filters and/or the antenna duplexer described above. In both the duplexer and the communication apparatus, the freedom of design can be increased and the size reduction can be achieved.
Other features and advantages of the present invention will become apparent from the following description of embodiments of the invention which refers to the accompanying drawings, in which like references denote like elements and parts.